In Radiation Oncology, a linear accelerator is used to deliver radiation beams to the patient from a rotating gantry.
One of the critical elements of the accuracy of the treatment is the geometrical accuracy of the gantry rotation. The point in space that defines the center of rotation must be determined before putting the system into use and must be verified routinely as part of a quality assurance program.
In order to determine the point in space that corresponds to the rotational axis, mechanical front pointers have been routinely used (see FIG. 1).
The Mechanical Front Pointer attaches to the treatment head and physically points to a location in space with a very accurate tip (approximately 1 mm). With the Front Pointer attached, the gantry can be rotated through a full 360° rotation while observing the tip of the Mechanical Front Pointer. The position of the Mechanical Front Pointer tip can then be adjusted to minimize the path of travel of the tip. Once this is done the Front Pointer tip is positioned at the center of rotational gantry.
One main problem with Mechanical Front Pointers is that they require the user to document (usually with a piece of paper that is placed adjacent to the tip of the Front Pointer) the tip path, and then iteratively make adjustments until the tip path is as tight as possible. Doing so introduces many errors due to parallax between the paper and the Front Pointer tip and human error in placing a dot with a pen onto the paper. Not only is this process subject to human error, but it is very time consuming.